Rubber modified monovinylidene aromatic polymers and fabricated articles prepared therefrom

ABSTRACT

The present invention is a rubber modified monovinylidene aromatic polymer composition which can be used in conventional thermoforming or other highly orienting forming or shaping processes to produce tough, cost effective, transparent containers or other packaging materials.

This application is a continuation in part (CIP) of prior applicationSer. No. 10/965,531, filed Oct. 14, 2004, now U.S. Pat. No. 7,208,547.

The present invention relates to improved rubber modified monovinylidenearomatic polymers.

Rubber modified monovinylidene aromatic polymers, such as rubbermodified or high impact polystyrene (HIPS), cost effectively provide arange of physical properties that can be balanced to suit numerousapplications. These types of polymers are frequently used in food anddrink containers and packaging products which can be produced in avariety of methods including molding and thermoforming from sheetmaterial. For use in these applications, such polymers require a balanceof properties including good impact strength, tensile strength andtransparency properties. Currently, the preferred polymer resins forthese applications are generally prepared by blending a separatestyrene-butadiene (SB) block copolymer rubber, often called a “K Resin,”with an unmodified polymer matrix, usually referred to as generalpurpose polystyrene or GP PS. This blended product has the rubbercomponent dispersed in the matrix polymer in a mostly co-continuousmorphology. These blends can provide a sufficient balance of toughnessand transparency in some thermoformed articles but, depending upon therequirements in specific applications, have been found to have importantand significant shortcomings in a number of areas including their cost,processability, recyclability, gel formation, surface gloss,printability and taste/odor.

Attempts have also been made to obtain the improved balance oftoughness, processability and transparency needed for thermoformingapplications in various rubber-modified resins where the rubbercomponent is added during the polymerization process. In such processes,often referred to as mass or solution polymerization, the rubbercomponent is added in solution, is formed into particles grafted withsome of the monovinylidene aromatic polymer, crosslinked during theprocess, and ending up as dispersed particles having various types ofparticle morphologies and containing varying amounts of monovinylidenearomatic polymer trapped or occluded therein.

In EP 167,707, translucent rubber-modified polystyrene is prepared bypolymerizing styrene with a polybutadiene (representing 5-50% of thetotal butadiene content) and a linear butadiene-styrene AB blockcopolymer in the presence of a peroxide radical initiator and mercaptansat elevated temperature to provide a dispersed soft phase (rubberparticles) having an average particle diameter of 0.02 to 0.8 micronsand provide improved notched impact strength and translucence.

U.S. Pat. No. 6,221,471 teaches polymer blends comprising a solutionpolymerized rubber modified monovinylidene aromatic polymer and aconjugated diene copolymer rubber, which can be used to produce costeffective, transparent packaging materials with a good toughness. Thoseblends comprise dispersed particles having a core/shell morphology and avolume average particle size of from 0.1 to 0.4 microns and a dispersedconjugated diene copolymer rubber.

There is always the need, however, for resins having improvedcombinations of toughness, transparency and processability forsuccessful use in thermoforming applications. Therefore, there remains aneed for polymer compositions which can produce cost effectivetransparent packaging and containers, which can be used in food,beverage and other applications and can be used in conventionalthermoforming lines with the ability to be readily recycled.

SUMMARY OF THE INVENTION

The present invention is a rubber modified monovinylidene aromaticpolymer composition comprising: a) a monovinylidene aromatic polymermatrix; b) from about 1.5 to about 8 percent by weight rubber (based ontotal diene content in the composition) dispersed as crosslinked rubberparticles having primarily a core/shell morphology and a volume averageparticle size of from about 0.1 to about 1.5 microns; where from about40 to about 90 volume percent of the rubber particles have diameters ofless than about 0.4 microns and from about 10 to about 60 volume percentof the rubber particles have diameters between about 0.4 and about 2.5microns and the rubber comprises a conjugated diene block copolymerrubber comprising from about 15 to about 60 percent by weightmonovinylidene aromatic monomer block; and c) optionally from 0.1 to 4weight percent mineral oil.

In other embodiments, the composition according to the present inventioncomprises from about 2 to about 6 percent by weight rubber, at leastabout 80 percent of the rubber particles have a core/shell morphologyand the volume average particle size is from about 0.2 to about 1micron. In further aspects, in polymer compositions according to theinvention from about 50 to about 80 volume percent of the rubberparticles have diameters of less than about 0.4 microns and from about20 to 50 volume percent of the rubber particles have diameters of fromabout 0.4 to about 1.2 microns and the rubber is a blend comprisingconjugated diene block copolymer rubber with from about 2 to about 25weight percent of conjugated diene homopolymer rubber.

The present invention also includes such polymer compositions wherein atleast 90 volume percent of the total rubber particles have core/shellmorphology and the volume average particle size of the rubber particlesis from 0.2 to 0.6 micron.

In further preferred embodiments, the present invention is a sheetsuitable for thermoforming produced from the polymer composition anddesirably having a thickness up to about 4.5 mm as well as a filmproduced from the polymer composition and desirably having a totalthickness of up to about 0.25 mm.

Improved thermoformed articles and thermoforming process are alsoprovided. In a particular aspect, the present invention is an improvedthermoforming process using sheet material prepared from the subjectresins comprising the steps of (a) positioning a heated sheet preparedfrom such resin to a position over a mold cavity; (b) stretching/drawingthe softened sheet material into a mold cavity with air pressure and/orvacuum and/or mold plug to provide the shape of the molded article andcut the article from the sheet; (c) removing the thermoformed articlefrom mold; and (d) recycling the remaining sheet material withadditional amount of such polymer and providing in sheet form to athermoforming process.

The novel polymer compositions of this invention can be used inconventional thermoforming applications to produce cost effective,tough, transparent packaging systems or containers for food, beveragesand other markets.

DETAILED DESCRIPTION OF THE INVENTION

Monovinylidene aromatic homopolymers and copolymers (collectivelyreferred to as “polymers” or “(co)polymers”) are produced bypolymerizing monovinylidene aromatic monomers such as those described inU.S. Pat. Nos. 4,666,987, 4,572,819 and 4,585,825, which are hereinincorporated by reference. The monovinylidene aromatic monomer suitablefor use in the matrix polymer component, graft polymerization onto therubber and copolymerization into the copolymer rubber component ispreferably of the following formula:

wherein R′ is hydrogen or methyl, Ar is an aromatic ring structurehaving from 1 to 3 aromatic rings with or without alkyl, halo, orhaloalkyl substitution, wherein any alkyl group contains 1 to 6 carbonatoms and haloalkyl refers to a halo substituted alkyl group.Preferably, Ar is phenyl or alkylphenyl, wherein alkylphenyl refers toan alkyl substituted phenyl group, with phenyl being most preferred.Typical monovinylidene aromatic monomers which can be used include:styrene, alpha-methylstyrene, all isomers of vinyl toluene, especiallyparavinyltoluene, all isomers of ethyl styrene, propyl styrene, vinylbiphenyl, vinyl naphthalene, vinyl anthracene and the like, and mixturesthereof with styrene being the most preferred. The monovinylidenearomatic monomer suitable for use in the matrix polymer component can becopolymerized with minor amounts of up to about 30 weight percent of oneor more of a range of other copolymerizable monomers. Preferredcomonomers include nitrile monomers such as acrylonitrile,methacrylonitrile and fumaronitrile; (meth)acrylate monomers such asmethylmethacrylate or n-butyl acrylate; maleic anhydride and/orN-arylmaleimides such as N-phenylmaleimide. If a copolymer is employed,preferred copolymers contain at least about 80, preferably at leastabout 90 weight percent monovinylidene aromatic monomer based on weightcopolymerizable monomer.

The monovinylidene aromatic polymer matrix component of the resincompositions according to the present invention typically have asufficiently high weight average molecular weight (Mw) to provide therequired level of processability and mechanical properties in thecomposition, which is typically a Mw of at least about 100,000,preferably at least about 120,000, more preferably at least about130,000 and most preferably at least about 140,000 grams per mole(g/mol).

The monovinylidene aromatic polymer component of the resin compositionsaccording to the present invention typically have an Mw that is lessthan or equal to about 260,000, preferably less than or equal to about250,000, more preferably less than or equal to about 240,000 and mostpreferably less than or equal to about 230,000 grams per mole (g/mol) inorder to provide sufficient processability.

The monovinylidene aromatic polymer component of the resin compositionsaccording to the present invention typically have a number averagemolecular weight (Mn) of at least about 30,000 preferably at least about40,000, more preferably at least about 50,000 and most preferably atleast about 60,000 grams per mole (g/mol).

The monovinylidene aromatic polymer component of the resin compositionsaccording to the present invention typically have an Mn that is lessthan or equal to about 130,000 preferably less than or equal to about120,000, more preferably less than or equal to about 110,000 and mostpreferably less than or equal to about 100,000 grams per mole (g/mol).

Along with these values for Mw and Mn, the ratio of Mw/Mn, also referredto as the polydispersity or molecular weight distribution, shoulddesirably be at least about 2, preferably greater than or equal to 2.3and less than or equal to 4, preferably less than or equal to 3. As usedherein, the terms Mw and Mn for the monovinylidene aromatic polymerrefer to weight and number average molecular weights as determined bygel permeation chromatography using a polystyrene standard forcalibration.

The rubber used for the rubber modified monovinylidene aromatic polymersor copolymers of the present invention is a conjugated diene copolymerrubber or a blend further comprising a minor amount of a rubberyconjugated diene homopolymer. The conjugated diene in both rubbers istypically a 1,3-alkadiene, preferably butadiene and/or isoprene, mostpreferably butadiene.

Suitable conjugated diene copolymer rubbers are also known and includecopolymers containing, in polymerized form and on a elastomeric polymerweight basis, from 40 to 90 percent of a conjugated diene, preferably a1,3-alkadiene monomer such as butadiene or isoprene, and from 10 to 60weight percent of one or more monoethylenically unsaturated comonomerssuch as the monovinylidene aromatic monomers described above, includingstyrene, and/or the ethylenically unsaturated copolymerizable monomersincluding acrylonitrile, methacrylonitrile, methyl methacrylate, ethylacrylate and the like. Preferred copolymer rubbers contain at leastabout 45 weight percent of the 1,3-alkadiene monomer, preferably atleast about 50 weight percent, preferably at least about 55 weightpercent and more preferably at least about 60 weight percent and lessthan or equal to about 85 weight percent 1,3-alkadiene monomer,preferably less than or equal to about 80 weight percent of the1,3-alkadiene monomer. Correspondingly, the amount of the copolymerizedmonoethylenically unsaturated comonomer(s) in the copolymer rubber ispreferably at least about 10 weight percent, preferably at least about15 weight percent and more preferably at least about 20 weight percentand less than or equal to about 50 weight percent, more preferably lessthan or equal to about 45 weight percent and more preferably less thanor equal to about 40 weight percent.

These copolymer rubbers are preferably block copolymers such as thetypes AB, ABA, tapered AB and ABA, and copolymers with varying degreesof coupling including branched or radial (AB)n and (ABA)n copolymers,where A represents a polymerized monovinylidene aromatic monomericcompound and B represents a polymerized conjugated diene, and “n” is awhole number greater than 2. Other resinous block copolymers withdifferent sequences of A and B blocks can also be used as the copolymerrubber in the present invention. Preferred types are diblock rubbers(AB-type), but ABA or mixtures of ABA and AB, can also be used.

The A blocks could be polymerized styrene, alpha-methylstyrene,4-methylstyrene, 3-methylstyrene, 2-methylstyrene, 4-ethylstyrene,3-ethylstyrene, 2-ethylstyrene, 4-tertbutylstyrene, 2,4-dimethylstyreneand condensed aromatics such as vinyl naphthalene and mixtures thereof.Presently preferred is styrene. The rubbery B block could bepolybutadiene, polypentadiene, a random or tapered monovinylidenearomatic/conjugated diene copolymeric block, polyisoprene, a random ortapered monovinylidene aromatic-isoprene copolymeric block, or mixturesthereof. Presently preferred is butadiene and/or isoprene.

The preferred copolymer rubber has a weight average molecular weight(Mw) of at least about 100,000, preferably at least about 150,000 g/moland less than or equal to about 350,000, preferably less than or equalto about 300,000, more preferably less than or equal to 250,000. Theweight average molecular weight values for this and any other rubbercomponent are referred to as true weight average molecular weights andare measured with Tri Angle Light Scattering Gel PermeationChromatography.

Such block copolymers and methods for their production are well known inthe art, and are described in G. Holden, et al; THEMOPLASTIC ELASTOMERS2ND EDITION; Hanser/Gardner Publications, Inc., 1996, ISBN1-56990-205-4, pages 48-70. They are also described in H. Hsieh and R.Quirk, ANIONIC POLYMERIZATION: PRINCIPLES AND PRACTICAL APPLICATIONS,Marcel Dekker Inc., 1996, ISBN 0-8247-9523-7, pages 307-321, and475-516.

Preferably the conjugated diene copolymer rubbers used in thecompositions according to the invention have solution viscosities in therange of from about 5 to about 100, preferably from about 20 to about 80centipoise (“cP”); and cis contents of at least about 20%, preferably atleast about 25% and more preferably at least about 30% and less than orequal to about 99%, preferably less than or equal to about 55% and morepreferably less than or equal to about 50%. Buna BL 6533 T brand rubberand other similar rubbers are preferred for use. For reference andcomparison herein and with other references, solution viscosity units ofcentipoise are equivalent to milli pascal seconds (mPa.s) and a pascalsecond (Pa·s) is equal to 10 poise or 1000 centipose.

Conjugated diene homopolymer rubbers can also be advantageously used inthe compositions according to the present invention. Conjugated dienehomopolymer rubbers that are suitable for use are generally known,having a second order transition temperature of 0° C. or less,preferably −20° C. or less. Preferably they have a solution viscosity inthe range of from about 20 to about 250 centipoise (cP), preferably fromabout 80 to about 200, and a cis content of at least about 20%,preferably at least about 25% and more preferably at least about 30% andless than or equal to about 99%, preferably less than or equal to about55% and more preferably less than or equal to about 50%. Preferably theyhave a weight average molecular weight of from about 100,000 to about600,000 g/mol, preferably from about 150,000 to about 500,000 g/mol(measured by Tri angle Light Scattering Gel Permeation chromatography).These polymers can be linear or branched. Diene 55 brand rubber(trademark of Firestone) and other similar rubbers are preferred foruse.

The rubber content of the final rubber modified monovinylidene aromaticpolymer composition of the present invention is measured by countingonly diene content from the copolymer rubber component and not includingany copolymerized monovinylidene or other non-diene monomer that is partof the copolymer rubber. In order to obtain the desired level ofmechanical strength and toughness in low strain rate deformations (suchas tensile strength at 5 mm/min deformation rate) and specificallyelongation at rupture in the tensile strength test, the rubber modifiedmonovinylidene aromatic polymer compositions according to the presentinvention will typically have a rubber content of at least about 1.5weight %, preferably at least 2.0 weight %, more preferably at least 2.5% and most preferably at least 3 weight % percent by weight, based onthe total weight of the rubber modified monovinylidene aromatic polymercomposition. In order to provide the desired transparency, the rubbermodified monovinylidene aromatic polymer compositions according to thepresent invention will typically have a rubber content of less than orequal to about 8 weight percent, preferably less than about 6 weightpercent, more preferably less than about 5.5 weight percent, morepreferably less than or equal to about 5 weight percent, more preferablyless than or equal to about 4.5 weight percent and most preferably lessthan or equal to about 4 weight percent by weight diene, based on thetotal weight of the rubber modified monovinylidene aromatic polymercomposition.

It has been found that minor amounts of a conjugated diene homopolymerrubber can contribute to the mechanical performance of the resin andspecifically to reaching the levels of elongation at rupture for theapplication. In order to obtain this result, if used, the conjugateddiene homopolymer rubber content of the rubber component in thecompositions according to the present invention will typically be atleast about 2 percent by weight based on total diene content, preferablyat least 4 weight percent, more preferably at least 6 weight percent,and most preferably at least 8 percent by weight, based on the totaldiene weight. In order to maintain a lower average particle size andavoid poor transparency, if used, the conjugated diene homopolymerrubber content of the rubber component in the compositions according tothe present invention will typically be less than or equal to about 25percent by weight, preferably less than or equal to about 20 percent byweight, more preferably less than or equal to about 16 weight percent,and most preferably less than or equal to about 12 percent by weight,based on the total weight of the rubber component.

The melt flow rate of the polymer composition needs to provide goodextrusion and thermoforming processability. This typically requires amelt flow rate as measured by ISO-1133 under Condition G (200 degrees C.and 5 kg) of at least about 1, preferably at least about 2, morepreferably at least about 3 and most preferably at least about 4 g/10min and generally less than or equal to about 15, preferably less thanor equal to about 13, more preferably less than or equal to about 12 andmost preferably less than or equal to about 10 g/10 min.

The rubber modified monovinylidene aromatic polymers or copolymers areproduced by known methods of polymerizing monovinylidene aromaticmonomers in the presence of a pre-dissolved elastomer, examples of whichare described in U.S. Pat. Nos. 3,123,655 and 4,409,369, which areincorporated by reference herein. In particular, a preferred rubbermodified monovinylidene aromatic polymer used in the blends of thepresent invention and the method for making, is disclosed in U.S. Pat.No. 5,491,195, incorporated herein by reference.

The conjugated diene copolymer rubber or, if using a blend of tworubbers, both starting rubber materials, is/are preferably dissolved inthe monovinylidene aromatic monomer and/or optional process diluent andsupplied to a reactor configuration suitable for polymerization of theresin composition. Preferably the rubber solution is fed into a seriesof agitated plug flow reactors with a series of temperature controlledzones. Preferably, mineral oil and a diluent are also fed to thereactor(s). In a preferred embodiment, a chain transfer agent can alsobe added to the reaction mixture before the first zone or into the firstor second zones of the first reactor.

Although thermal (heat initiated) polymerization conditions arepreferred, it is also possible to use low levels of a polymerizationinitiator selected from the known initiators including the peroxideinitiators including the peresters, for example, tertiary butylperoxybenzoate, tertiary butyl peroxyacetate, dibenzoyl peroxide, anddilauroyl peroxide, the perketals, for example, 1,1-bis tertiary butylperoxycyclohexane, 1,1-bis tertiary butyl peroxy-3,3,5-trimethylcyclohexane, and di-cumyl peroxide, and the percarbonates; and photochemical initiation techniques. These initiators may be employed in arange of concentrations dependent on a variety of factors including thespecific initiator employed, the desired levels of polymer grafting andthe conditions at which the mass polymerization is conducted. If used,from about 50 to about 300, preferably from about 100 to about 200,weight parts of the initiator are employed per million weight parts ofmonomer.

During the polymerization, the rubber will be grafted with aromaticpolymer and dispersed into particles. The dispersed, grafted rubberparticles will typically have, per one part by weight of the rawmaterial un-grafted rubber polymers, at least about 1, preferably atleast about 2, more preferably about 3 parts by weight monovinylidenearomatic polymer and less than or equal to about 7, preferably less thanor equal to about 6, and more preferably less than or equal to about 5parts by weight of monovinylidene aromatic polymer grafted thereto andoccluded therein.

The majority of the rubber particles dispersed within the rubbermodified monovinylidene aromatic polymer matrix need to have acore/shell particle morphology, preferably at least 70 percent, morepreferably at least 80 percent, more preferably at least 90 percent. Asused herein the term core/shell morphology means that the rubberparticles have a thin outer shell and contain a single, centeredocclusion of the matrix polymer. This type of particle morphology iscommonly also referred to in the art as “single occlusion” or “capsule”morphology. In contrast, the terms “entanglement” or “cellular”morphology refer to various other, more complex rubber particlemorphologies that are known in the art and have structures that can bedescribed as “entangled”, “multiple occlusions”, “labyrinth”, “coil”,“onion skin” or “concentric circle”. As used herein, the core/shellrubber particle percentage is a numerical percentage based on counting500 particles in transmission electron micrographic photos (TEM's).

The core/shell rubber particles in the compositions according to thepresent invention will typically have a volume average size of at leastabout 0.01 micron, preferably at least about 0.1 micron and morepreferably at least about 0.3 micron and typically less than or equal toabout 2.0 micron, preferably less than or equal to about 1.5 micron andmore preferably less than or equal to about 1 micron, and mostpreferably less than or equal to about 0.6 micron. In addition to havingan average particle size in this range, it has also been found to beimportant to obtain a relatively broad particle size distribution wherethe majority of the particles are smaller and having only a limitedamount of larger particles. In particular, it has been found desirableto have a distribution where from about 40 to about 90 volume percent ofthe particles have diameters less than about 0.4 microns.Correspondingly, it has been found desirable to have a distributionwhere from about 10 to about 60 volume percent of the particles havediameters greater than about 0.4 microns and less than about 2.5,preferably from about 15 to about 55 volume percent and more preferablyfrom about 20 to about 50 volume percent of the particles, havediameters greater than or equal to about 0.5 microns and less than orequal to about 2.5 microns. Preferably, for this component of relativelylarge particles, the specified percentage amounts of the particles havediameters less than about 2 microns, more preferably less than or equalto about 1.5 micron and more preferably less than or equal to about 1.2microns. When it cannot be avoided, the compositions according to theinvention may contain limited amounts of somewhat larger particleshaving some of these other particle morphology types but this willdetrimentally affect the transparency of the final product.

As used herein, the volume average particle size refers to the diameterof the rubber particles, including all occlusions of monovinylidenearomatic polymer within the rubber particles. Average particle size andother rubber particle statistics and percentages can be measured bymeans of the Beckham Coulter: LS230 light scattering instrument andsoftware. The use of this equipment for this application is discussed inthe manufacturer's instructions and literature and in the JOURNAL OFAPPLIED POLYMER SCIENCE, VOL. 77 (2000), page 1165, “A Novel Applicationof Using a Commercial Fraunhofer Diffractometer to Size ParticlesDispersed in a Solid Matrix” by Jun Gao and Chi Wu. Preferably, withthis equipment and software, the optical model used to calculate therubber particle size and distribution statistics is as follows: (i)Fluid Refractive Index of 1.43, (ii) Sample Real Refractive Index of1.57 and (iii) Sample Imaginary Refractive Index of 0.01.

Other additives may be included in the compositions of the presentinvention, such as mineral oil, other plasticizers and the like. Forexample, it has been found that, in appropriate amounts, mineral oilprovides further improvements in the elongation at rupture of the finalproduct. The rubber modified monovinylidene aromatic polymercompositions according to the present invention will typically have amineral oil content of at least about 0.4 weight percent, preferably atleast 0.6 weight percent, more preferably at least 0.8 weight percentand most preferably at least 1.0 weight percent, based on the totalweight of the rubber modified monovinylidene aromatic polymercomposition. In order to obtain the desired level of transparency in thefinal product, the rubber modified monovinylidene aromatic polymercompositions according to the present invention will typically have amineral oil content of less than or equal to about 3 weight percent,preferably less than or equal to about 2.8 weight percent, morepreferably less than or equal to about 2.6 weight percent and mostpreferably less than or equal to about 2.4 weight percent, based on thetotal weight of the rubber modified monovinylidene aromatic polymercomposition.

The material is devolatilized and pelletized according to the knowntechniques. As known to those skilled in this area of technology, thedevolatilization conditions can be used to adjust the crosslinking ofthe rubber particles and to thereby provide optimized mechanicalproperties.

Preferably, for use in thermoforming applications, it has been founddesirable for resin compositions according to the present invention tohave values for tensile yield in megaPascals (MPa) of at least 20;preferably at least 26 and more preferably at least 32, (ASTM D-638)

It is also preferable for thermoforming applications if resincompositions according to the present invention have values forelongation at rupture (in %) in the resin compositions of at least 10%;preferably at least 15% and more preferably at least 20%. (ASTM D-638)

It has also been found to be desirable for thermoforming if resinsaccording to the present invention have tensile modulus values (in MPa)in the resin compositions of at least 1800, preferably at least 2,000and more preferably at least 2,200. (ASTM D-638).

It is particularly desirable for thermoforming applications for theresins according to the present invention, prior to thermoforming, toprovide haze values (in %) in 0.5 mm injection molded plaques of lessthan 60%, preferably less than 50% and more preferably less than 40%.(ASTM D-1003-95). Then, as discussed further below, the haze values inthermoformed articles having a wall thickness of about 200 microns willpreferably be less than or equal to about 20 %, more preferably lessthan or equal to about 15%, more preferably less than or equal to about10%. For thicker or thinner wall thicknesses, a 200 micron value can becalculated based on the turbidity values for the oriented polymercompositions as calculated from the haze and thickness values measuredon the thermoformed article or part thereof.

The novel polymer compositions of this invention are preferably prepareddirectly as the product of a solution or mass polymerization process asdiscussed above. Alternatively, these directly prepared products can beused in a blend with additional amounts of one or more other, separatelyprepared monovinylidene aromatic monomer polymers or copolymers toengineer materials with a somewhat different range of cost/propertybalances as might be needed for some packaging or containerapplications. Alternatively, the final products could be prepared byblending an amount of separately prepared monovinylidene aromatic(co)polymer with an amount of rubber-modified polymer component havingthe rubber particle morphology and distribution needed to provide thefinal product rubber content in the proper range. Examples of othermonovinylidene aromatic monomer polymers or copolymers which can beblended with or used to provide the compositions according to theinvention include, but are not limited to, general purpose polystyrene,high impact polystyrene, monovinylidene aromatic copolymers (such aspoly(styrene-acrylonitrile), styrene/diene block copolymers,styrene/diene random copolymers, vinyl aromatic/olefin monomerinter-polymers (such as ethylene/styrene copolymers).

The most surprising and beneficial effect of the compositions accordingto the present invention is the transparency/toughness combination thatis obtained after fabrication and particularly when sheets are extrudedand thermoformed into shaped articles. The products according to thepresent invention provided significantly better and unexpected resultsin this regard than were expected from known correlations between thehaze of a molded resin plaque and the transparency obtained in thinnerthermoformed articles. Although it is known and expected thatthermoforming or other processes providing a similar, high degree ofpolymer orientation improve the transparency of rubber-modified resinsbased on occlusion-containing rubber, the resins according to theinvention provide much better transparency in thermoformed articles thanwould have been expected based on the thermoforming of otherwise similarrubber-modified, solution or mass polymerized monovinylidene aromaticpolymers. Other processes providing a similar, high degree of polymerorientation include extrusion blow molding and injection stretch blowmolding of bottles, containers or other hollow articles and biaxialstretching or bubble blowing of oriented films.

According to optical laws known to those skilled in this art, thetransmittance can be expressed as a function of turbidity (“τ”) andsample thickness (“x”) according to the following equation:

${Transmittance} = {{100\frac{I}{Io}} = {100\mspace{14mu}{\mathbb{e}}^{{- \tau}\; x}}}$Haze  % = 100 − Transmittance  %

where I is the transmitted intensity and I₀ the incident intensity andthe turbidity (τ) is a parameter inherent to the material.

For most materials, therefore, the haze of molded parts should onlychange as a function of part thickness. This does not apply to highimpact polystyrene systems due to their heterogeneous nature. Rubberparticles orient under shear fields during thermoforming and some otherprocesses and the rubber wall thins as a result of this orientation. Ifthe material is cooled down fast enough, the particles will not havetime to relax and this will change the rubber domain morphology as wellas the value of τ for this type of system. An example of this behaviourwill be shown in the examples section.

The materials of this invention are different from standard, mass orsolution polymerized high impact polystyrene in that the rubber particlesize distribution as specified is relatively broad and the majority ofthe rubber particles have a core-shell morphology. In contrast,conventional HIPS resins tend to have a relatively narrow particle sizedistribution and have predominantly or at least a larger percentage ofcellular, multi-occlusion particle structures. Core-shell particles inthe compositions according to the invention are crosslinked to thedegree that they will stretch but not break under shear fields (i.e.during thermoforming or other highly orienting process). Their thinnerwalls (as a result of higher compatibility coming from the presence ofcopolymer rubbers) will become even thinner but remain intact to providethe needed mechanical and tensile strength properties. It is believedthat the oriented rubber morphology, because there are so few, if any,multi-occlusion particles in the system (cellular morphology), is veryclose to a co-continuous distribution of very thin ribbons of rubber.The very thin shell walls have better light transmittance than wouldresult with thicker walls and definitely better than if there wereresidual cellular or multi-occlusion particles which do not distributein this fashion when oriented such as during thermoforming.

When used in thermoforming and other highly orienting processingconditions, the compositions according to the invention provideexcellent combinations of transparency and toughness and, veryimportantly, toughness under low strain rate conditions (from 0.1 mm/minto 2000 mm/min) that correspond to the typical usage of food andbeverage packaging and containers.

In other aspects of the present invention, therefore, there are providedimproved thermoformable sheets or films, improved thermoformed articles,an improved process for preparing thermoformed articles, an improvedprocess for providing extrusion blow molded articles, and an improvedprocess for providing injection stretch blow molded articles. In theseaspects of the invention, the resin compositions as described aboveprovide surprising combinations of processability, recyclability,toughness, gloss, transparency and other properties compared to priorart and commercially available resins and the articles prepared fromthem. In another embodiment, the compositions described above can beused to prepare multilayer sheet or film having, for example, threelayers comprising a core or middle layer of a composition describedabove and outer layers of another resin located on each side.

Thermoformed articles, such as drink glasses and containers for foodssuch as dairy products, prepared from the resins according to thisinvention are surprisingly tough and transparent allowing see-throughinspection of any materials contained by such thermoformed articles.Single or multilayer thermoformable sheet can be produced using knowntechniques in the art, including but not limited to known extrusiontechniques. Single or multilayer films can be produced from thecompositions described above using known cast, tenter frame or blownfilm techniques.

The thicknesses for sheet, blanks or other preformed starting materialsfor thermoforming applications and equipment are typically from 0.2 to4.5 millimeters (mm) preferably from 0.3 to 3.75 mm, more preferablyfrom 0.4 to 2.0 mm, and most preferred from 0.50 to 1.5 mm. Such sheetscan be further processed by thermoforming into articles which have goodimpact strength and transparency. Thicknesses of less than 0.2 mm canalso be achieved and used in applications where thin gauge materials aredesired, such as transparent lids.

The thickness of films that can advantageously be prepared from resinsaccording to this invention are typically less than 0.25 mm, preferablyfrom 0.012 to 0.20 mm, more preferably from 0.018, more preferably from0.020, more preferably from 0.023 and most preferably from 0.025 up to0.15, preferably to 0.13, more preferably to 0.10, more preferably to0.05, more preferably to 0.04, more preferably to 0.03 and mostpreferably to 0.025 mm.

The thermoformable sheets can be produced by the known flatdie/calendaring mono- or coextrusion processes. This sheet can then bethermoformed into containers either “in-line” directly and withoutcooling after sheet production or “off-line” where the sheet isprepared, cooled and provided to the thermoformer. The thermoforming ineither case can then be done by appropriate standard plug and moldthermoforming machines including “form-fill-seal” container lines. Thissheet (monolayer or multilayer) can be used to produce form-fill-sealand other packaging having good transparency, barrier and toughnessproperties, as well as preformed packages or containers by standardthermoforming equipment.

The known thermoforming processes generally employ the following mainsteps:

-   -   1. Heat sheet to a temperature in range of 125 to 170° C. (if        not using in-line process where sheet is already/still in a heat        softened condition), depending upon (co)polymer heat softening        temperature;    -   2. Position sheet in molding area over a mold cavity;    -   3. Start stretching/drawing the softened sheet material into a        mold cavity with air pressure and/or vacuum and/or mold plug.        This can be done sequentially or in combinations of two or more        at once. Preferably the stretching is started with positive air        pressure on one side with minimized contact between the plug and        the polymer material. A “plug” is a moveable mold part that        forces the polymer sheet into a mold cavity and can be provided        with a coating of Teflon or a silica material to reduce friction        and sticking of the polymer.    -   4. Final shaping of the article with the plug using plug speed,        force and shape to provide the final shape and uniform thickness        in the thermoformed article and cut it away from the sheet.    -   5. Plug removed, mold opened and part removed, mold opens, part        removed    -   6. The remaining sheet material, with holes corresponding to the        removed, thermoformed areas (referred to as the “skeleton”) is        removed and preferably recycled.

The resin sheets of the present invention are typically thermoformed athigh speeds using well known techniques such that the polymers areoriented as discussed above and the thermoformed articles exhibit goodtransparency and toughness properties. The sheet thermoformingtemperature is typically below 170° C. and is preferably between 125 and150° C. The thermoforming process drawing speed (strain rate) isgenerally above 200 mm/second, and is preferably between 220 and 340mm/second as measured by using the thermoforming cycle time in cyclesper minute and knowing the time and height of the plug assist and theheight or depth dimension of the thermoformed article made each cycle.

Thermoformed articles prepared using this process are surprisinglytransparent and tough when the draw down ratio of the article is atleast about 0.1, preferably at least about 0.4, more preferably at least0.6 and most preferably is at least 0.8 and is less than or equal toabout 10, preferably less than or equal to about 7, preferably less thanor equal to 1.8, more preferably less than or equal to 1.6. As usedherein, the draw down ratio is the ratio of the height or depthdimension of the article to the greatest diagonal or diameter dimensionof the mold cavity area.

Regarding the haze measurements on thermoformed articles, it isrecognized that there will be thickness variations in most thermoformedarticles that will result in variations in haze as measured or observedat those locations. One major advantage provided according to thepresent invention is the generally low haze and also the low turbiditygradient observed when the above-described compositions are thermoformedinto articles. As shown in the haze/turbidity/thickness relationshipabove, with low turbidity, there will be much less haze differential inlocations of the thermoformed articles where there are thicknessvariations. Depending on the thickness of the thermoformed article, itwill typically have haze values of less than 20 percent, preferably lessthan 15 percent and most preferably less than 10 percent (measured in anarea about 200 microns in thickness) once thermoforming is complete. Thehaze and transparency values were determined with a Hunter LabTristimulus Colorimeter Model D25P-9 with glass test standard numbered425 in accordance with ASTM Method D1003-95.

The following examples are provided to illustrate the present invention.The examples are not intended to limit the scope of the presentinvention and they should not be so interpreted.

EXAMPLES

The rubber component(s) shown in Table 1 below were dissolved togetherin styrene in the ratios shown in Table 3 below to prepare the indicatedresin products. Also included in this feed stream was 2.5 weight percentmineral oil (70 centistokes kinematic viscosity). The rubber blendcontent in the feed and the feed rates of styrene and rubber to thereactor are calculated with a target of producing an end rubber-modifiedpolystyrene product containing 4% of butadiene.

TABLE 1 Rubber Components Conjugated Diene Conjugated Diene CopolymerRubber Homopolymer Rubber Property Buna BL 6533 T Diene 55 (Bayer)(Firestone ™) Styrene Content/% 40 0 Vinyl Content/% 9 11 Cis Content/%38 38 Mooney Viscosity 45 70 ML1 + 4 100 C. Solution Viscosity 40 170(5.43% in toluene)/cP Other AB Block copolymer Generally linear

Sample compositions were produced in a continuous process using threeagitated reactors working in series. The rubber feed solution, mineraloil, ethyl benzene, styrene and the rest of the additives were suppliedto the first reactor at a rate of 750 g/h. The rubber concentrations andEB concentrations in the feed are given in Table 3 as a weightpercentage of total feed. The final conversions to polymer are alsoprovided in this Table. The rubber composition in the final product canbe calculated from the conversions and the rubber content in the feed.In all cases, 0.1% of total feed of Antioxidant Irganox 1076 was addedto provide levels of about 1200 ppm in the final product. Each reactoris divided into 3 zones that control temperature independently. Thetemperature profile used was: 125, 130, 135, 143, 149, 153, 157, 165,170° C. The agitation used in the first reactor is given in Table 3, inthe second was 50 rpm and in the third 25 rpm. Different levels of achain transfer agent (n-Dodecyl Mercaptan or nDM) as summarized in Table3 below, were added in the first reactor, into the second zone of theseries of nine reactor zones.

A devolatilizing extruder was used to flash out the residual styrene andethylbenzene diluent and crosslink the rubber. The temperature profilein the extruder was: start of barrel: 240° C., medium zone of barrel:240° C.; final zone of barrel: 240° C. and screw temperature: 220° C.

Table 3 summarizes the raw materials, reaction conditions and theproperties of the HIPS materials obtained with these rubbers. Table 3also compares these resins with two comparative blend resins containinga rubber component and a standard general purpose polystyrene or “GPPS”(STYRON 678E brand PS) having a weight average molecular weight of240,000 and a MFR of 10.5 grams/10 min at condition G. The rubbercomponent in the first comparative resin was 35 weight percent ofstandard HIPS (STYRON A-TECH 1200) containing about 8.5 weight percentof a low cis polybutadiene rubber (based on butadiene) in the form ofcellular particles having a volume average rubber particle size of about2 microns. The rubber component in the second blend resin was about 50weight percent of STYROLUX 3G35 brand SB block copolymer rubbercontaining about 73 weight % styrene and 27 weight % butadiene andhaving a melt volume rate of 15 grams/10 minutes at condition G (200degree, 5 kg).

The test methods used were:

-   MFR—ISO-1133-   PS Matrix molecular weight distribution—PS calibration Gel    Permeation Chromatography.-   Rubber Particle Size—Light scattering using an LS230 apparatus and    software from Beckman Coulter.-   Tensile Yield—ISO-527-2-   Tensile Elongation—ISO-527-2-   Tensile Modulus—ISO-527-2-   Haze (injection molded plaque)—ASTM D-1003-95 using a 0.5 mm    injection molded plaque-   Haze (thermoformed article)—ASTM D-1003-95 using a flat, 1 inch×1    inch square piece free from topological defects cut from the    thermoformed part and having a thickness of about 200 microns.-   Gloss (injection molded plaques)—ASTM D-523-89 using the indicated    Table 2 injection molding conditions.-   Part thickness was measured with an ABSOLUTE Digimatic Caliper from    Mitutoyo.

TABLE 2 Injection moulding conditions Haze Gloss Screw Diameter/mm 35 25Holding Pressure/Bars 500-900 50 Holding time/s  5-15 3 MoldTemperature/C. 50 40 Polymer melt temperature/C. 235 235 Cooling time/s25 25 Back Pressure/Bars 125 50

TABLE 3 Resin Production Examples Comparative Blend of GP Comparativeand SB block Blend of GP copolymer Example 1 and HIPS rubber Diene 55concentra- 0.3 N/A N/A tion/% in feed Buna 6533 concentra- 4.5 N/A N/Ation/% in feed Styrene Balance N/A N/A EB Concentration in 7 N/A N/A thefeed/% feed Conversion to polymer/ 80 N/A N/A % feed nDM addition toZone 100 N/A N/A 2/ppm First agitator rpm 80 N/A N/A Butadiene Content/%3.8 3 12.2 In final product Matrix Mw/g/mol 189,000 210,000 N/A MatrixMn/g/mol 87,000 80,000 N/A Mw/Mn 2.2 2.6 N/A Vol Avg Particle 0.32 2.0N/A size μm Particle morphology >90 <50 N/A (% core/shell) Particle Vol% <0.4 65 0 N/A μm diameter Particle Vol % 0.4- 35 100 N/A 2.5 μmdiameter Diene Homopolymer- 0.38 3 N/A wt % in Product Diene Copolymer -5.6 — N/A wt % in Product Mineral Oil - 2.5 3.3 3.2 wt % in ProductM.F.R./g/10 min 8.4 8.5 >10 Vicat/° C. 99.8 99 <90 Tensile Yield/MPa33.8 25.5 26 Elongation at 31 40 55 Rupture/% Tensile Modulus in 24802650 2000 MPa Haze % (injection 33 66 8.5 moulded plaque) Gloss (60°angle) >100 70 >100

It is apparent from this table that a very desirable finalmechanical/optical property balance is obtained according to theinvention.

Thermoformed parts were prepared from the Example 1 resin according tothe present invention and the two blend resins as summarized in Table 3above. This will illustrate the optimized combinations of propertiesaccording to the invention and particularly, the improvement in opticalproperties as a result of the fabrication process.

Extruded sheets having a thickness of 1.1 mm were made on a flat sheetdie and vertical roll stack extruder at a melt temperature of about 210°C. The extruded sheet was then thermoformed into yogurt containershaving a capacity of approximately 125 grams. The cup was cylindricallyshaped with straight walls and no taper conicity, a wall height (ordepth) of 64.6 mm and a diameter of 54.6 mm at the circular opening,having a drawdown ratio of 1.2. The cup was thermoformed on a labmachine that is similar to and represents the set up of an industrialmachine. The single cavity machine was equipped with a clamping frame,aluminum female mold cavity, vertical displacement syntactic foam plug,contact heating (135-145° C.) and regulated air positive pressure (3-6bars). The 1.1 mm sheet was fed manually to produce a cup with wallthickness in the body of the cup between 130 and 225 micron, yielding athickness distribution of 0.7 micron (calculated as follows: (sheetthickness/wall thickness)-(sheet thickness/bottom thickness). The cupsappeared transparent visually and the haze was measured to be 6% at awall location where the thickness was about 200 microns.

This resin according to the invention provides significantly betterthermoforming processes and/or thermoformed article properties than thecomparative blends of GP and K-resin or commercially available HIPS, asshown in the Table below, and, in contrast to thermoforming resins basedon blends of GP and SB block copolymers, is easily recyclable. Thetoughness of a cup is evaluated by downwardly compressing the bottom ofan upside down cup at a rate of 10 mm/s and measuring the total forcerequired to press the cup bottom downward a distance of 3 mm. A cupmeasured by this method is also noted to be “brittle” if, after the 3 mmcompression, the cup wall is cracked or broken.

TABLE 4 Key properties of Thermoformed Cups Haze - τ - Haze - 200 micronthermo- 0.5 mm thermo- τ - IM formed Sample IM plaque formed part plaquepart Toughness Example 1 35 6 0.0008 0.0003 No brittle cups GP/HIPS 6615 0.0021 0.0008 No brittle (35/65) cups GP/SB 8.5 5 0.00018 0.00018 Nobrittle block cups copolymer rubber (50/50)

A further rubber-modified monovinylidene aromatic film resin accordingto the present invention was prepared generally according to the processas described above having the specific features and composition asindicated below and was fabricated into oriented films.

TABLE 5 Film Resin Production Example nDM addition to Zone 2/ppm 100Matrix Mw/g/mol 195,000 Matrix Mn/g/mol 85,000 Mw/Mn 2.3 Vol AvgParticle size μm 0.35 Particle morphology (% core/shell) >90 ParticleVol % <0.4 μm diameter 65 Particle Vol % 0.4-2.5 μm diameter 35 DieneHomopolymer- wt % in Product 0.38 Diene Copolymer - wt % in Product 5.6Butadiene Content/% In final product 3.8 Mineral Oil - wt % in Product2.0 M.F.R./g/10 min 7.0 Vicat/° C. 101 Tensile Yield/MPa 30 Elongationat Rupture/% 25 Tensile Modulus in MPa 2480 Haze % (injection mouldedplaque) 33 Gloss (60° angle) >100

Cast extruded sheet having a thickness of 1.47 mm was prepared from theresin described above using a flat sheet coathanger die equipped with avertical 3-roll stack. Melt temperature was held at about 210C. Theextruded sheet was subsequently biaxially oriented into films of varyingthickness using a laboratory T. M. Long Film Stretcher. Prior tostretching, the extruded sheet was heated to a temperature ofapproximately 120° C. for 3 minutes. The sheet was then biaxiallyoriented using a simultaneous stretching mode at a rate of 0.5 inchesper second. The films were stretched by approximately the same amountsin both the machine and transverse directions and the indicated stretchratios are averages of the individual machine and transverse directionstretch ratios (stretched dimension to unstretched dimension).

For the 1.47 mm sheet, successful biaxial orientation could beaccomplished at bidirectional stretch ratios from 2.5× to 5×. Although120° C. was found to be an optimal temperature for biaxial orientation,a wide operating window for temperatures in the range of 95 to 125° C.and for stretch ratios could be employed.

Optical properties of haze and clarity were measured for the films,using ASTM test methods D-1003 and D-1746, respectively. The filmsamples obtained demonstrated excellent optical character, as evidencedby the data shown in the following table.

Stretch Ratio Film Thickness Haze Clarity (times) (mm) (%) (%) 2.5 0.249 47 3 0.16 5.6 60 3.5 0.12 5.1 78 4 0.09 3.4 82 4.5 0.07 2.3 85 5 0.062.4 86

The improved optical properties of the biaxially orientedrubber-modified films are believed to be a result of the modification ofthe rubber phase morphology that occurs after the biaxial orientationprocess. Coming out of the resin production and in the cast orunoriented state, the crosslinked rubber particles exist as primarilysubmicron-sized, primarily core/shell morphology globules which scatterlight and detract from the optical transparency. In the biaxiallyoriented state, the rubber phase becomes drawn out and more dispersedand approximates nearly a lamellar or thread-like morphology, with therubber particles existing as very thin, elongated domains.

These biaxially oriented films were also tested for mechanicalproperties, such as ultimate tensile strength and ultimate elongation,using ASTM D-882. As evidenced by the data in the table below, the filmexhibits improved tensile strength and elongation with increasingstretch ratio, and hence amount of biaxial orientation.

Stretch Ratio Film Thickness Ultimate Tensile Ultimate (times) (mm)Strength (MPa) Elongation (%) 2.5 0.24 40.3 34 3 0.16 44.5 34 3.5 0.1241.8 26 4 0.09 44.4 78 4.5 0.07 49.1 91 5 0.06 56.1 83

This result of improved transparency, while maintaining an acceptablebalance of physical/mechanical properties, is not similarly observed asa result of otherwise similar biaxial orientation of otherrubber-modified resins.

1. A film having a total thickness of up to about 0.25 mm produced froma rubber modified monovinylidene aromatic polymer compositioncomprising: a) a monovinylidene aromatic polymer matrix; b) from about1.5 to about 8 percent by weight rubber (based on total diene content inthe composition) dispersed as crosslinked rubber particles havingprimarily a core/shell morphology and a volume average particle size offrom about 0.1 to about 1.5 microns; where from about 40 to about 90volume percent of the rubber particles have diameters of less than about0.4 microns and from about 10 to about 60 volume percent of the rubberparticles have diameters between about 0.4 and about 2.5 microns and therubber is a blend comprising a conjugated diene block copolymer rubbercomprising from about 15 to about 60 percent by weight monovinylidenearomatic monomer block with from about 2 to about 25 weight percent ofconjugated diene homopolymer rubber based on the total weight of rubberin the composition; and c) optionally from 0.1 to 4 weight percentmineral oil.
 2. A film according to claim 1 having a total thickness offrom 0.012 to 0.20 mm.
 3. A film according to claim 1 wherein the rubbermodified monovinylidene aromatic polymer composition comprises fromabout 2 to about 6 percent by weight rubber.
 4. A film according toclaim 1 wherein the rubber modified monovinylidene aromatic polymercomposition, at least about 70 percent of the rubber particles havecore/shell morphology and the volume average rubber particle size isfrom about 0.2 to about 1 micron.
 5. A film according to claim 1 where,in the rubber modified monovinylidene aromatic polymer composition, fromabout 50 to about 80 volume percent of the rubber particles havediameters of less than about 0.4 microns and from about 20 to 50 volumepercent of the rubber particles have diameters of from about 0.4 toabout 1.2 microns.
 6. A film according to claim 1 where, in the rubbermodified monovinylidene aromatic polymer composition, the monovinylidenearomatic matrix polymer is polystyrene.
 7. A film according to claim 1where, in the rubber modified monovinylidene aromatic polymercomposition, at least 90 percent of the total rubber particles havecore/shell morphology and the volume average particle size of the rubberparticles is from 0.2 to 0.6 micron.